“Free electrons” are the electrons which are controlled, meaning the electrons may be extracted from the electron source, accelerated, and directed in a specified direction. For generation of “free electrons”, the electron source should provide conditions for efficient emission of electrons from a cathode. Direct extraction of electrons from a metal surface by applying an electric field, defined as an auto electron emission requires a high electric field and is known to be limited in the current density.
As an alternative approach to the direct extraction of electrons, electron sources use triggers in which the current pulse passes through a gas, converting it into highly ionized state, called a plasma, which is a good emitter as it has no potential barrier preventing the emission of electrons.
The main electrical triggering technique used in conventional electron generating devices is surface flashover in which a current pulse flows in a gas along a dielectric surface that assists the discharge ignition. For low pressure gases or vacuum, this surface supplies the discharge media (absorbed gas molecules, surface defects, sharp edges, inclusions of foreign materials, etc.). In this technique, discharge literally flashes over the dielectric surface. Disadvantages of such a trigger of the discharge ignition are (1) limited emission of electrons and (2) short lifetime. Both of these disadvantages are related to the fact that current tends to flow through a narrow channel between metallic electrodes of the device. As a result, volume of the plasma channel is limited, and accordingly so is the emitted electron current. Additionally, due to the current confinement into the channel dielectric surface erosion limits lifetime of the trigger to as little as 106 pulses.
Other electron emitters make use of some specific material such as dielectrics (or ferro electrics) with high dielectric constant ε˜1000 to overcome the problems associated with limited electron emission and shortened lifetime. A typical electron emitter consists of a dielectric plate mounted between two electrodes positioned on opposite sides thereof, such that one electrode covers the side of the plate completely, while another, e.g., a face electrode, leaves significant area of the side of the dielectric plate uncovered.
Presence of the high ε dielectric facilitates performance of such an electron emitter. First, the emitter can work at lower voltages as it amplifies and focuses an electric field on the edge of the face electrode. Second, it provides distribution of the surface/over-current over larger area as it creates an integrated distributed capacitor that the discharge current tends to charge. Third, due to the distributed nature of the discharge, erosion of the surface is reduced, and lifetime of such a device is elongated.
This design of electron generators/triggers is suitable for large area electron switches or electron guns. However, due to their distributed nature, the design is less effective for some devices where the generated electrons should be concentrated in narrow beams. In this case, only a small fraction of the trigger area, which is located in the vicinity of the device/beam axis, works effectively.
A more effective generator/trigger device than that found in the prior art is needed for generation of focused electron beams.